Solid-state imaging apparatus or imaging readers, that have been configured either as handheld, portable scanners and/or stand-mounted, stationary scanners each having a presentation window, or as vertical slot scanners each having a generally vertically arranged, upright window, or as flat-bed or horizontal slot scanners each having a generally horizontally arranged window, or as bi-optical, dual window scanners each having both generally horizontally and vertically arranged windows, have been used in many venues, such as supermarkets, department stores, and other kinds of retailers, libraries, parcel deliveries, as well as factories, warehouses and other kinds of industrial settings, for many years, in both handheld and hands-free modes of operation, to electro-optically read by image capture a plurality of symbol targets, such as one-dimensional symbols, particularly Universal Product Code (UPC) bar code symbols, and two-dimensional symbols, as well as non-symbol targets, such as driver's licenses, receipts, signatures, etc., the targets being associated with, or borne by, objects or products to be processed by the imaging readers. In the handheld mode, a user, such as an operator or a customer, held the imaging reader and manually aimed a window thereon at the target. In the hands-free mode, the user slid or swiped a product associated with, or bearing, the target in a moving direction across and past a respective window in a swipe mode, or momentarily presented the target associated with, or borne by, the product to an approximate central region of the respective window, and steadily momentarily held the target in front of the respective window, in a presentation mode. The choice depended on the type of the reader, or on the user's preference, or on the layout of the venue, or on the type of the product and target.
The imaging reader included a solid-state imager (also known as an imaging sensor) with a sensor array of photocells or light sensors (also known as pixels), which corresponded to image elements or pixels over a field of view of the imaging sensor, and an imaging lens assembly for capturing return light scattered and/or reflected from a target being imaged over a working range of distances, and for projecting the return light onto the imaging sensor to initiate capture of an image of the target as pixel data. The imaging sensor was configured as a one- or two-dimensional charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) device, and included associated circuits for producing and processing an electrical signal corresponding to a one- or two-dimensional array of the pixel data over the field of view. The imaging sensor was controlled by a controller or programmed microprocessor that was operative for processing the electrical signal into data indicative of the target being imaged and, when the target was a symbol, for processing and decoding the symbol.
The known imaging lens assembly typically comprised a plurality of lenses of different sizes and optical powers, such as a classical Cooke triplet that allowed elimination of most of the optical distortion or color aberration at the outer edge of the lenses. The Cooke triplet typically comprised a negative flint glass lens in the center of the lens assembly with a crown glass lens on each side of the center lens. The lenses were held in a lens holder and axially arranged along an optical axis. An aperture stop having a rotationally symmetrical aperture, e.g., a circular aperture, or a non-rotationally symmetrical (or asymmetrical) aperture, e.g., a rectangular or elliptical aperture, centered on the optical axis, was typically located between one of the side glass lenses and the center glass lens. In the Cooke triplet, the sum of all the lens curvatures multiplied by the indices of refraction of the lenses was typically designed to be zero, so that the field of focus was flat (zero Petzval field curvature).
Traditionally, each lens of the Cooke triplet was made of glass for improved thermal stability, because glass has a relatively lower coefficient of thermal expansion and a relatively lower refractive index variation over temperature, as compared to plastic, for example. Hence, an all-glass lens design was typically used to minimize focal shift over an operating temperature range. To provide the known imaging lens assembly with a wide angle field of view, e.g., greater than 50 degrees, it was known to add a fourth glass spherical lens of negative optical power in front of the Cooke triplet. Sometimes, to improve the imaging performance, a fifth glass spherical lens was added.
However, the all-glass imaging lens design comprised of three, four, or five or more glass lenses was relatively heavy and expensive. It was possible to reduce the number of glass lenses by configuring aspherical surfaces on a respective glass lens. However, the manufacture of aspherical surfaces on glass lenses by machining and polishing was challenging and costly. Aspherical surfaces could be readily and inexpensively molded on a plastic lens, which was also lighter than a corresponding glass lens. Nevertheless, despite the lighter weight and lower fabrication cost of the plastic lens, the thermal instability and focal shift were unacceptable in many electro-optical reading applications.
To simultaneously achieve both effective thermal stability and effective color aberration correction, it was known to configure an imaging lens assembly by positioning a pair of plastic lenses having substantially no optical power at one side of an aperture stop, and by positioning a pair of glass lenses having substantially all the optical power of the imaging lens assembly at an opposite side of the aperture stop. The plastic lenses provided optical aberration compensation, while the glass lenses provided thermal stability.
As advantageous as this hybrid glass/plastic imaging lens assembly was, the magnitudes or absolute values of the optical powers at both sides of the aperture stop were imbalanced, because the optical power of the plastic lenses was very low, e.g., near zero, while the optical power of the glass lenses was very high. This optical imbalance made it very difficult to correct distortion and other odd aberrations, e.g., astigmatism, coma, etc. to acceptable levels. In addition, the known plastic lenses tended to be overly sensitive to manufacturing and assembly tolerances, thereby increasing the cost and time of manufacture and assembly. The field of view of the known hybrid glass/plastic lens assembly was also limited in size and was curved.
Accordingly, it would be desirable to provide a compact, lightweight and inexpensive, imaging lens assembly of high thermal stability, with minimal optical distortion or color aberration, with less sensitivity to manufacturing and assembly tolerances, and with a wider and flatter field of view, especially useful in portable and mobile applications where size, weight and cost are at a premium.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.